check conditions of  make  for equality of variables with an empty value
instead of check definition.
A defined but empty variable under all these conditions results errors.

.fop.xconf needed for building pdf.

This initial page might lack a lot, don't be shy to blame me!
(thanks ldv@ for having skimmed it through though)

A bunch of intermediate files would be sitting around otherwise.

This one is long overdue as I keep forgetting to update
the published copy of generated documentation all the time.

Note that you must pass DOCS_PUBLISH (as a local directory
or host:dir suitable for rsync/ssh) via make arguments,
environment or ~/.mkimage/profiles.mk file.

The initial sketch did work but was somewhat redundant
while lacking the knob conveniently change output directory
as well as means to get it cleaned up.

Thanks torabora@ for the tip along with reference to his
http://altlinux.org/asciidoc page.

Docs should live happily together, after all,
and toplevel should be more or less uncluttered :)

Adjusted/rewrote as appropriate as well.